The present invention relates to a liquid level detector and more particularly to a sensor for sensing the levels of brake fluid for providing a warning signal when the amount of brake fluid falls below a predetermined level.
The problems of detecting the presence of hydraulic fluid in moving vehicles are numerous. These problems are caused by vehicle acceleraton or deceleration which causes the fluid to splash and often signal a false alarm.
This problem is further caused by lateral shift of the fluid due to change in attitude of the vehicle.
The detection problem is further aggravated when the vehicle is used in environments having a wide range of temperatures. The change in temperature has a substantial effect on the resistivity of the fluid. The resistivity of the brake fluid increases as the temperature decreases. This often causes the signalling of a false alarm by brake fluid detection devices which detect fluid levels by measuring the electrical resistance through the fluid.
Another problem in the detection of the presence of brake fluid is caused by ionization of the fluid by devices which utilize the resistivity of the fluid. This ionizaton is frequently caused by detection devices which measure liquid levels as a function of the resistivity of the fluid by applying an electrical current through that fluid. A high electrical current causes ionization of the fluid which in turn causes a breakdown in the structure of rubber seals which thereby reduces their effectiveness.
Various prior art mechanical devices have been used for detecting liquid level. Such devices frequently employ a float on the liquid which moves up and down as a function of the level of the liquid. Such devices do not provide self-check circuitry. Furthermore, they do not have the reliability of electronic switching devices. Furthermore, these devices are particularly susceptible to false alarms due to change in vehicle attitude and/or acceleration.
Various prior art electronic devices have been employed to detect fluid level. Such devices frequently employ field effect transistors (FET) or high-gain bipolar transistors. Such devices have been characterized by excessive probe current requirements which cause ionization of the fluid. The circuitry of such devices generally does not perform well over wide ranges of temperature. Generally, these devices do not have protection against transient and reverse supply potential and do not have self-check capability.